After describing this species, C. H. Muller later (1951, 1970) concluded that it represented a hybrid between Quercus emoryi and Q . gravesii . The extreme forms (e.g., the type specimens) of Q . robusta and Q . gravesii are easily differentiated, but these two taxa appear to occupy the ends of a morphologic continuum. Muller's recent view, however, is that Q . robusta deserves species status. The origin of this taxon is still worthy of study.